Method for avoiding interference radiation of am transmitters used in digital transmission

ABSTRACT

In digital transmission using AM transmitters, non-linear distortions occur because of the band limitation in the amplitude branch and also in the RF-branch, resulting in unwanted spurious emissions. In order to meet the requirements of the ITU, it is required to achieve constant shapes of the amplitude response, the phase response and the delay-time characteristic in the amplitude branch and also in the RF-branch and to compensate for the propagation time difference between the two branches. In the RF-branch, the required bandwidth can be achieved by suitable selection of the quality of the resonant circuits. For the amplitude branch, a pre-equalization with an inverse transfer function is applied to compensate for the non-linearities. To apply the method, it is required for the amplitude response and the delay-time characteristic to be determined with measuring techniques and to be stored. The transfer function for the equalizing network is determined by an inverse Fourier transform to the measured values. The network is connected in series upstream of the amplitude branch. As far as the propagation times of the amplitude branch and the RF-branch are constant, the propagation time difference can be compensated for in the digital modulator by delaying the signal having the smaller propagation time. The compensation of the non-linearities by a pre-equalization with an inverse transfer function can also be used for the RF-branch.

SPECIFICATION

[0001] The present invention relates to the field of broadcast transmitters which will be converted from analog amplitude modulation (AM) to digital modulation as digitalization moves forward.

[0002] In this context, the intention is for the hitherto usual transmitter types, non-linear AM transmitters featuring an RF input (radio frequency) and an audio input, to continue in use. The reasons for this are as follows:

[0003] AM transmitters internally operate in switched mode and therefore have efficiencies which are better by a factor of 3 than those of linear transmitters which are otherwise usually used for digital transmission, for example, in the case of DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting). This results in a saving of operating costs.

[0004] it is easier to convince broadcasters to convert from analog to digital if no great investments come up in the preliminary stages.

[0005] The digitalization of AM broadcasting is seen as the only chance to preserve these frequency ranges and the technology used therein in the long term. For implementation purposes, the consortium “Digital Radio Mondiale” was founded, see “Rundfunktechnische Mitteilungen” [Broadcasting Newsletter], 43rd year, 1999, issue 1, pages 29-35.

[0006] The use of a non-linear AM transmitter for digital modulation requires a special operating mode of the transmitter. The modulated digital signal is generated by two partial signals (I and Q), which are orthogonal to each other. The I-signal (“in phase”) is modulated onto a cosine oscillation having the frequency Ft (carrier frequency). The Q-signal (“quadrature”) is modulated onto a sine oscillation having the same frequency Ft. The sum of both modulated oscillations produces the complex modulated data signal (cosine 0 180 degrees, sine 90∓90 degrees). The modulated I/Q-signal is shaped by filters in such a manner that it has exactly the prescribed curve shape with the desired bandwidth.

[0007] For non-linear operation, it is required for the modulated I/Q-signal to be converted in such a manner that the two signals amplitude signal (A-signal) and phase-modulated carrier signal (RF-P) result therefrom which are suitable for proper control of the AM transmitter. Then, at the output of the AM transmitter, the modulated I/Q-signal is generated again with higher power.

[0008] The modulated I/Q-signal corresponds to a Cartesian representation. The Cartesian representation is converted to a polar representation with amplitude and phase. In this manner, the amplitude signal (A-signal) is obtained to control the AM transmitter at the audio input. A phase-modulated radio frequency (RF-P signal) is generated from the initially resulting phase signal (P-signal). Advantageously, the RF-P signal can also be directly obtained without the intermediate step via the P-signal. In this manner, the signals are obtained that are required for controlling the AM transmitter:

[0009] amplitude signal (A-signal)

[0010] phase-modulated RF signal (RF-P signal)

[0011] The A-signal is fed into the modulator input (audio input) of the AM transmitter, and the RF-P signal is used for HF-type control of the transmitter. In the transmitter output stage, the two signals A & RF-P are multiplicatively combined, forming the high frequency digital output signal.

[0012] Due to the required conditioning process, both the A-signal and the RF-P signal obtain far larger bandwidths than the one the digital signal originally had and is intended to have again at the output of the transmitter.

[0013] Older modulators are frequently not able to provide the increased bandwidths (3-5 times the bandwidth in the AM mode of the transmitter) because they were not designed for this. When using only the limited bandwidth that “older” transmitters have available in the modulator section, then this results in considerable out-of-band and spurious emissions. These have the property that they have only a very small gradient in the spectrum and therefore interfere with quite a number of adjacent channels.

[0014] Thus, the band limitation in the amplitude branch (A-branch) and also in the RF-branch results in spurious emissions whose shoulder distance in relation to the spectrum of the data channel does not or only very inadequately meet the requirements of the ITU.

[0015] The object of the present invention is to eliminate the non-linear distortions arising because of the band limitation in the amplitude branch and also in the RF-branch of the AM transmitter so as to meet the ITU limits during the emission of the digital signal.

[0016] In particular, the band limitation in the A-branch and also in the RF-branch causes the following transmission errors:

[0017] the amplitude response in the A-branch is not constant but decreases with increasing frequency,

[0018] phase response in the A-branch is not linear and, consequently, the propagation time is not constant,

[0019] the amplitude response and the delay-time characteristic of the RF-branch are not constant,

[0020] signal propagation times of the A-branch and of the RF-branch are different (delay).

[0021] The different signal propagation times between the amplitude branch and the RF-branch have to be accurately adjusted to values smaller than 1 microsecond in the digital modulator. However, the adjustment works only if the propagation times both in the amplitude branch and in the RF-branch are constant over the frequency. However, coupled or associated with the decrease of the amplitude response with increasing frequency, the band limitation in both branches also causes a non-linearity of the phase, that is, non-constant propagation times.

[0022] Since in the RF-branch, the bandwidth is determined by the frequency limits of the resonant circuits, in particular of the driver stage, it is possible, through suitable selection of the quality of the resonant circuits, to adjust such a large bandwidth that the then still resulting out-of-band and spurious emissions become negligible compared to those resulting from other influences.

[0023] For the amplitude branch, the intention is to remedy the band limitation and the resulting distortion of the amplitude response and the delay-time characteristic by applying a pre-equalization with an inverse transfer function to the amplitude signal to thereby achieve an optimum compensation of the non-constant amplitude response and delay-time characteristic.

[0024] The inverse shape of the transfer function is limited to the frequency range from 0-20 KHz. This value is determined by the fact that in the A-branch a bandwidth of about 3-5 times the audio frequency bandwidth or about 3-5 times the bandwidth of the digital I/Q signal of 4.5 KHz (long wave and medium wave) or 5 KHz (short wave) is required and that, due to the limitation to this range, no instability can arise. In the case that transmission channels (two to four adjacent channels) are bundled at the transmitter end, the numerical values mentioned for the bandwidth are increased according to the bundling factor.

[0025] To apply this method, it is required for the curve shapes of the amplitude response and the delay-time characteristic of the A-branch to be determined with measuring techniques and to be available in stored form. Then, the mathematical values of the transfer function for the absolute value and the phase are determined by numerical interpolation, the shape of the phase being determined by integration over the measured shape of the propagation time. The impulse response to the measured shapes of amplitude and phase is determined by an inverse Fourier transform. In this manner, the impulse response yields the coefficients for the filter for the pre-equalization of the amplitude signal so that a compensation of the non-linearities in the A-branch is achieved.

[0026] The network used for the pre-equalization can be a filter having a FIR structure (finite impulse response) in which the signal can be tapped downstream of each of the stages of the chain of delay elements so that the tapped signals can be weighted and added up according to the desired impulse response.

[0027] The network with inverse shapes of amplitude, phase or propagation time is connected in series upstream of the amplitude branch of the transmitter.

[0028] Since the antenna matching of the transmitter acts back upon the amplitude response, the phase response and the delay-time characteristic of the A-branch, different shapes arise, depending on the matching conditions. Therefore, it suggests itself to measure these shapes for usual matching conditions and to store the results. This makes it possible to select the appropriate shapes for the compensation, for example, in the case of weather-related variations in impedance of the antenna matching. The selection criterion for this can be provided by the measuring device for the antenna matching which is present in the transmitter anyway and which, possibly, needs to be modified and adapted for that purpose. In this manner, the compensation is ideally adapted to the practical transmitter operating conditions. Through corrections of the stored curve shapes, it is also very easily possible to control an optimum for the achieved shoulder distance.

[0029] The compensation of the non-constant amplitude response and delay-time characteristic by a pre-equalization with an inverse transfer function can also be used for the RF-branch.

[0030] As far as the amplitude branch and the RF-branch have constant propagation times, the propagation time difference between the two branches can be compensated for using the simple method of delaying the signal having the smaller propagation time. 

1. A method for avoiding spurious emissions in AM transmitters for digital transmission, in which the digital modulation for controlling the AM transmitters is converted into an amplitude signal and a phase-modulated RF signal, wherein in the amplitude branch, the non-linear distortions are compensated in that a pre-equalization with inverse shapes is applied for the amplitude response and the delay-time characteristic, to this end, the amplitude response and the delay-time characteristic are measured in the amplitude branch and stored, the inverse transfer function is determined from the measured shapes of the amplitude response and the delay-time characteristic by an inverse Fourier transform, an equalizing network is dimensioned for the inverse transfer function, the equalizing network is connected in series upstream of the amplitude branch, in the RF-branch the bandwidth of the resonant circuits, through suitable selection of the quality, is adjusted in such a manner that a linear shape is achieved for the amplitude response and the delay-time characteristic, and the propagation time difference between the amplitude branch and the RF-branch is compensated for in the digital modulator by delaying the signal having the smaller propagation time.
 2. The method as recited in claim 1, wherein the inverse transfer function for the equalizing network is limited to the frequency range of 3-5 times the bandwidth of the digital I/Q signal to avoid instability and overdriving of the transmitter.
 3. The method as recited in claim 1, wherein the amplitude response and the delay-time characteristic of the amplitude branch are measured and stored in different conditions for the reactive antenna matching so that the pre-equalization can be optimally adapted in the case of weather-related variations in impedance.
 4. A method for avoiding spurious emissions in AM transmitters for digital transmission, in which the digital modulation for controlling the AM transmitters is converted into an amplitude signal and a phase-modulated RF signal, wherein the compensation of the non-linear distortions in the amplitude response and the delay-time characteristic by a pre-equalization with an inverse transfer function can also be applied to the RF-branch. 